Charged particle beams, laser beams, and neutral particle beams are used in a variety of microfabrication applications, such as fabrication of semiconductor circuitry and microelectromechanical assemblies. The term “microfabrication” is used to include creating and altering structures having dimensions of tens of microns or less, including nanofabrication processes. “Processing” a sample refers to the microfabrication of structures on that sample. As smaller and smaller structures are fabricated, it is necessary to direct the beam more precisely.
One method of accurately positioning a beam is to place or mill a fiducial, that is, a reference mark, on the sample near an area of interest, and position the beam relative to the fiducial. The term fiducial is used broadly to include any type of reference mark. A beam is initially directed to image a fiducial and an initial offset to the desired location is determined. Subsequently, the beam is periodically directed to image the fiducial and the positioning of the beam to the desired location is corrected by determining an offset between the observed coordinates of the fiducial and the original coordinates of the fiducial. The offsets are then added to the beam positioning instructions so that the beam ends up at the desired location.
FIG. 1A shows a top view of a sample 100 including an area of interest 102 and a fiducial 104. Area of interest 102 is a portion of sample 100 where an imaging or milling operation is to be performed. For example, sample 100 may comprise a semiconductor wafer and area of interest 102 may comprise a particular integrated circuit feature that is to be imaged by a scanning electron microscope or focused ion beam to verify whether the feature has been manufactured to within specification. Typically, a charged particle beam is used to mill fiducial 104 in the surface of the sample near area of interest 102 so that the area of interest can be located quickly and easily among a surface having many different features. When subsequently imaged at an angle that is normal to the surface of sample 100 (“top down view”), fiducial 104 can be more readily or quickly identified by the operator of the instrument (or by automation software controlling the instrument) than the area of interest 102 itself can be identified.
However, fiducial 104 becomes less identifiable as fiducial 104 is imaged at angles that are not normal to the top surface of sample 100. For angles that are nearly parallel to the top surface of the sample 100, fiducial 104 may not be identifiable at all. The top surface of sample 100 is the surface that is opposite of the surface that is in contact with the sample stage holding sample 100. FIG. 1B shows a side view of sample 100 in which fiducial 104 cannot be seen. Positioning the beam for imaging or milling at this angle would require a second fiducial.